SHERMAN, Texas -- A charter bus carrying a Vietnamese Catholic group to an annual pilgrimage slammed onto its side and then skidded off a freeway early Friday, killing at least 15 people and injuring more than 40, authorities said.

Hospital officials confirmed Friday night that a woman died as a result of injuries she sustained in the bus accident. She is the 15th person to have been killed in the crash.

Authorities said 12 people died at the scene, and three others died at Dallas-area hospitals.

The private charter bus was traveling from Houston to Missouri when it skidded off U.S. 75 near the Texas-Oklahoma line at about 12:45 a.m.

An official with the Archdiocese of Galveston/Houston confirmed that those on the bus were from the Vietnamese Martyrs Church of Houston on their way to a mission trip in Carthage, Mo. The bus was also carrying people from various churches, the source said.

“I’ll tell you — there were very few walking wounded,” said Sherman Fire Chief Jeff Jones. “The injuries I saw were primarily crushing wounds, trauma, as you’d expect in a situation like this.”

The bus was one of several headed to the Marian Days Festival, an annual celebration in honor of the Virgin Mary.

The tragedy was the nation’s deadliest bus crash since 2004, when 15 people were killed in a wreck in Arkansas on their way to Mississippi’s casinos. In 2005 near Dallas, 23 people were killed when a bus carrying nursing home residents away from Hurricane Rita caught fire while in bumper-to-bumper traffic.

The bus left from Houston Thursday night the church official said. 11 News confirmed that the church chartered the bus from what had been called Angel Tours, but recently changed it's name to Iguala BusMex Inc. of Houston.

Iguala BusMex Inc. had applied in June for a federal license to operate as a charter but was still awaiting approval, according to online records.

The company recently filed incorporation papers, listing the same owner and address as Angel Tours Inc., which was forced by federal regulators to take its vehicles out of interstate service June 23 after an unsatisfactory review, records show. Details of the review were not in the online records.

Neither entity is currently authorized to operate as a carrier in interstate commerce, said John H. Hill, administrator for the Federal Motor Carrier Safety Administration.

Federal investigators said the blown right front tire on the bus, which was operating illegally, had become delaminated—what happens when the tread separates from the tire.

“If there is a loss of pressure and the tire becomes delaminated, it’s much more difficult to control the vehicle,” said Debbie Hersman, a National Transportation and Safety Board member.

Hersman said tires can be retreaded but not on a steer axle. She said the NTSB had assigned six investigators and a team leader to the case.

The driver was a 52-year-old who had a commercial license but whose medical certification had expired, Hersman said.

Members of the Our Lady of Lavang Church were also on the bus, the church official told 11 News.

A Sherman police spokesman said 15 of the about 55 people aboard the bus were killed.

Apparently, the bus careened off a guardrail and fell off the overpass onto a creek embankment below, officials on the scene said.

The first officers to respond described a horrific scene, with luggage, hand bags and pieces of the bus strewn amid a pile of bodies, some dead, some severely injured. There were cries for help and looks of shock, officers said.

Many passersby stopped and tried to help and some survivors climbed through broken windows. Others lay helplessly trapped inside the wreckage.

“There were people deceased from the front of the bus to the back of the bus,” said Officer Zachary Flores, one of the first on the scene.

Twelve adults died at the scene and another died at a Dallas hospital, Sherman police said.  The survivors ranged in age from elderly to children, police said. The bus driver was believed to have survived.

“You’ve got 50-something people laying everywhere,” said Officer Tony Walden, also among the first on scene. “I don’t even know how to describe it.”

Lt. Robert Fair said while the bus had a blown right tire, it was not clear if that is what caused the accident.

The bus is a 2002 MCI bus purchased recently from MCI Inc. of Dallas, said Fair. The bus had a paper license tag that was set to expire on Saturday.

A spokeswoman with the Wilson N. Jones Medical Center in Sherman said that the hospital was treating 19 people, one of which was critical and was being transported via helicopter to a Dallas hospital. Two other passengers were taken to Southeastern Oklahoma Medical Center in Durant, Okla.

Presbyterian Hospital in Allen, Texas was treating three "pediatric" patients, a hospital spokeswoman told 11 News.

Four critical patients were being treated at Parkland Hospital in Dallas.

Patients were also being treated at Baylor Medical Center and Methodist Dallas Medical Center.

Passenger Leha Nguyen, 45, said she thought the bus was speeding as it passed through Dallas just before midnight. People began dozing off, but then she heard a noise and screaming, and she opened her eyes.

“I see ... somebody was laying on my legs. A lady next to me, she had her arm crushed up. The lady who was on my left ... a man was on top of her,” she said at a news conference at a Sherman hospital where she had been treated.

She said nobody was wearing seatbelts. She said people were strewn all over, some were not moving and a television had fallen on one person.

“I think I’m the luckiest one out of most people,” Leha Nguyen said.

The Rev. Joseph Vu, a priest at the Vietnamese Martyrs Church and vicar for the 30,000 to 35,000 Vietnamese Catholics in the region, was not on the trip but arrived at a relief station set up for victims’ families at a church in nearby Denison.

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Authorities are trying to find out exactly what caused the crash.

“I’m going to tell people we don’t blame anybody,” he said.  “This happened like Katrina, like Challenger. What we can do is pray.” He added: “God will comfort them. Tell people to keep trusting in God. Do not blame anybody. Do not ask why. Now we just help each other to get through this.”

A sobbing Mary Nguyen, a member of the Vietnamese Martyrs Church for more than 10 years, learned that a close friend had died. “She was just a very good person,” she said at the Houston church.  “The church is like one big family here. We’re very close. We stick together.”

“We are here with them to pray for those who are lost and for God’s consolation in this time of grief and loss,” Cardinal Daniel N. DiNardo said before celebrating Mass. “The Vietnamese Catholic culture is very strong. A lot of those who have come here have been through a great deal just to get to this country. They’ve always preserved their Catholic faith. This is a trial. This is a challenge.”

DiNardo said the losses, which included church leaders, were “incomprehensible.”

Organizers of the festival in Missouri said the victims would be remembered at Mass and at various conferences during the gathering.

The identities of the victims were not immediately released.

WFAA-TV, Dallas Morning News and The Associated Press contributed to this report.